Rule of thumb is every 3 months for electric heads or sooner if the bristles are splayed - once they fray, they clean less and can be rough on gums. In hard-water areas, a brief weekly soak in a half white vinegar, half water mix helps dissolve mineral deposits; rinse well and let it air-dry uncapped. Keeping a light touch and rotating two heads so each fully dries between uses can also eke out a bit more life without losing effectiveness.
If you are on an ISP gateway, the default features can be rough on smart home gear... Band steering on by default, nightly self healing, and client isolation on guest can all break the Alexa path. Make a separate 2.4 only SSID for IoT and confirm that client isolation is off so your Echo and the plugs can see each other. Keep it plain WPA2 AES and fixed channel. If the gateway insists on doing auto optimization at night, turn that schedule off or move the IoT SSID to an attached access point or extender that you control. Stability over speed here.
